This invention relates, in general, to semiconductor packages, and more particularly, to ball-grid array semiconductor packaging.
Ball-grid array (BGA) semiconductor packages are well known in the electronics industry. BGA packages typically comprise a substrate, such as a printed circuit board, with a semiconductor die having a multitude of bond pads mounted to the top side of the substrate. Wire bonds electrically connect the bond pads to a series of metal traces on the top side of the printed circuit board. This series of metal traces is connected to a second series of metal traces on the back side of the printed circuit board through a series of vias located around the outer periphery of the printed circuit board. The second series of metal traces each terminate with a contact pad where a conductive solder ball is attached. The conductive solder balls are arranged in an array pattern and have a pitch on the order of 1.5 millimeters (mm). Typically, the semiconductor die and wire bonds are encapsulated with a molding compound.
In order to reduce manufacturing costs, the electronics industry is increasing the usage of perimeter BGA packages. In perimeter BGA packages, the conductive solder balls are arranged in an array pattern on the back side of the printed circuit board between the vias and the semiconductor die. The inner-most conductive solder balls typically are beneath or in close proximity to the outer edges of the semiconductor die.
Users of BGA packages, particularly in computer, communication, and personal electronic applications, are demanding smaller, lighter and thinner (less than 1.5 mm) BGA packages to support, among other things, portability. Also, users are requiring a smaller pitch distance between the conductive solder balls in order to support higher pin counts within the same package footprint area. In addition, users are requiring improved conductor routing to the package. Furthermore, users are pushing for BGA packages that are more reliable under increasingly demanding operating conditions.
Existing BGA packages have several disadvantages including a high profile on the order of 2.4 millimeters (mm); collapse of the solder balls at reflow to a height determined by ball and ball pad size; and a fatigue failure rate of the solder joints during temperature cycling based on the higher strain levels in solder joints with lower height to diameter ratios.
Accordingly, there exists a need for a BGA package that supports the electronics industry's demand for a perimeter solder ball array, that has a low-profile (less than 1.5 mm), that has a reduced susceptibility to solder fellow collapse, that has a reduced rate of solder joint failure under temperature cycling, and that is cost effective.